Ethylene-carbon monoxide interpolymers (ECO) are a known class of compounds which are typically prepared by high pressure, free-radical initiated polymerization reactions. See, for example, U.S. Pat. Nos. 2,495,292; 2,436,269; 2,495,255; 2,495,286; 3,248,359; 3,780,140; 4,024,325; and 4,024,326 which are incorporated herein by reference. It is also known to produce ECO interpolymers by coordination catalytic means as described in U.S. Pat. Nos. 2,641,590; 3,083,184; 3,530,109; 3,689,460; 3,694,412; and 3,835,123, which are incorporated herein by reference.
ECO polymers are known to have poor resistance to thermal degradation during post-polymerization processing. Viscosity changes have even been observed when the ECO polymers are stored for extended periods of time at ambient conditions (e.g. warehouse conditions). The changes in melt flow rate and melt tension of the ECO polymers can render them unacceptable for subsequent processing and use. The stability problems are particularly severe for ECO polymers having a high carbon monoxide content (e.g., about 10 mole percent).
U.S. Pat. Nos. 3,948,850; 3,948,832; 3,968,082 to Hudgin and 3,929,727 to Russell et al. describe compounds said to be useful for preventing gelling, formation of surface skin and color change of ECO polymers.
U.S. Pat. No. 4,139,522 to Lantos describes manganous salts of phosphorus oxy acids useful f or preventing ECO polymers from crosslinking during processing operations at elevated temperatures.
U.S. Pat. No. 4,954,548 to Klingensmith describes UV stabilization of ECO polymer compositions with an aromatic diamine and carbon black.
U.S. Pat. No. 4,814,378 to Lutz describes a polymer blend of an ECO polymer and a vinyl aromatic compound with an .alpha.,.beta.-unsaturated cyclic anhydride copolymer which is said to have improved processability and melt stability.
A persistent stability problem with ECO remains. As far as applicant is aware, no solution has been devised for effectively stabilizing ECO polymers against viscosity changes at ambient temperatures and above, and ECO polymers have been limited in their use. Accordingly, there remains a need for a method for producing ECO interpolymers which are viscosity stable at conditions of transportation, storage and use.